1. Field of the Invention
The present invention relates to a high temperature proton-conducting polymer membrane, a preparation method thereof, a membrane-electrode assembly using the same and a fuel cell containing the same. More particularly, it relates to a proton-conducting polymer membrane enabling fuel cell operation under high temperature and normal pressure condition, wherein sulfoalkyl or sulfoaryl groups are introduced between layers of metal phosphate and cation exchange groups are present in side chains, a preparation method thereof and a membrane-electrode assembly using the proton exchange membrane and a fuel cell containing the same.
2. Description of the Related Art
A fuel cell is an electricity-generating system that converts the energy generated from electrochemical reaction of fuel and an oxidizer into electrical energy. Recently, there has been increasing demands for the development of high-performance fuel cell offering good energy efficiency, functioning at high temperature and having good reliability which can effectively handle the environmental problems, exhaust of energy sources and application of fuel cell cars. Further, there has been a demand for the development of polymer membrane functioning at high temperature to improve efficiency of fuel cells.
Fuel cells are largely classified as follows: carbonate fuel cells functioning at high temperature (500 to 700° C.); phosphate fuel cells functioning at about 200° C.; and alkaline fuel cells and polymer fuel cells functioning in the temperature range of from room temperature to about 100° C.
Of these, polymer fuel cells are considered as clean energy source of the future that can replace fossil energy. They have good output density and energy transfer efficiency. Also, since they function at room temperature and can be prepared in small size, they may be used in various fields such as clean cars, household power generation systems, mobile communication devices, medical devices, military equipments and space equipments.
There are two representing types in polymer fuel cells: proton exchange membrane fuel cells (PEMFC), which directly use hydrogen gas as fuel, and direct methanol fuel cells (DMFC), which directly provide liquid ethanol to anode as fuel.
The PEMFC is a power generation system that generates DC electricity from electrochemical reaction between hydrogen and oxygen. FIG. 1 shows a schematic diagram of PEMFC.
In a PEMFC, a proton-conducting polymer membrane 11 is located between an anode and a cathode. To be specific, a PEMFC comprises a proton-conducting polymer membrane 11 made of solid polymer, which is about 50 to 200 μm thick; support layers 14, 15 which feed the reaction gas; catalyst layers 12, 13 wherein oxidation and reduction of the reaction gas takes place, which is respectively located in an anode and a cathode (hereinafter, anode and cathode are collectively termed as “gas diffusion electrodes”); and a carbon plate 16 having a gas injection hole and functioning as current collector. Catalyst layers 12, 13 are located on support layers 14, 15 of the gas diffusion electrodes. The support layers 14, 15 are made of carbon fiber or carbon paper and their surface are treated so that water transferred to the proton-conducting polymer membrane 11 and water generated from the reaction may penetrate with ease.
At the anode of the PEMFC, a hydrogen gas is reduced to protons and electrons. Thus produced protons are then transferred to the cathode after passing through the proton-conducting polymer membrane 11.
At the cathode, an oxygen molecule takes up electrons and is oxidized into oxygen ions. The oxygen ions react with the protons transferred from the anode to form a water molecule.
While the proton-conducting polymer membrane is an electrical insulator, it functions as medium transferring protons from the cathode to the anode and separates gaseous or liquid fuel from the oxidizing gas.
Accordingly, the proton-conducting polymer membrane should have good mechanical properties and electrochemical stability. In addition, it should be able to be formed into a thin sheet to improve mechanical properties and heat stability and lower resistance. Further, it should not expand much when there is liquid penetration.
Currently, as such polymer membrane, fluorine based membranes having fluorinated alkylene in the main chain and sulfonate groups at the end of the fluorovinyl ether side chains are used (e.g., products of Nafion and DuPont). However, they are too expensive to be used in fuel cells for cars. Further, the cell operation temperature is limited below 100° C. due to increase in membrane resistance by dehydration at high temperature. The current fuel cells cannot be operated at high temperature under normal pressure condition due to the dehydration of proton-conducting polymer membrane. They require external pressure of at least 3 atm for operation at high temperature.
Therefore, researches have been focused on various polymer materials and organic/inorganic composite materials having superior electrochemical properties and heat stability and are also capable of solving the above-mentioned problems. Typical examples are heat-resistant aromatic polymers such as polybenzimidazole, polyether sulfone and polyether ketone. However, these aromatic polymers are too rigid to be dissolved, so that they are difficult to be prepared in the form of a membrane.
Researches on preparing inorganic composite materials comprising highly hygroscopic silica are in the progress. However, they also have problems in electrical conductivity since inorganic materials cannot transfer protons, or only a few if any.